1. Field of the Invention
The present invention relates to a loop testable radio transmitter/receiver.
2. Description of the Related Art
In recent years, with the spread of the value added network, an economical radio communication equipment which enables easy installation working has been demanded. For the improved reliability of circuits for advanced information, the quick separation work and restoring work of failed equipment at trouble state is required.
Moreover, recently, the development of a small, enclosed, outdoor mountable radio communication system substantially built-in with an antenna is being required, such as radio communication equipment for communications between mobile communication base stations or offices, ultra-small earth station radio communication equipment for satellite communications, and the like. Such an outdoor radio communication equipment requires a self-diagnosis function for the loop testing to confirm and evaluate its performance at maintenance. However its structures and its general installation conditions do not allow to equip the looping tester outside the equipment body. Thus, it is necessary to arrange a circuit for receiving a self-transmission signal to the receiving input and for converting it to an intermediate frequency signal, inside the equipment body.
A radio transmitter/receiver, for example, as shown in FIG. 31, includes an indoor equipment 200, an outdoor equipment 201, and IF cables 202 interfacing the equipment 200 with the equipment 201. The indoor equipment 200 is formed of a modulator/demodulator 100 and a digital component 101. The outdoor equipment 201 is formed of a transmission frequency converter (up-converter) 1, a transmission filter 2, a receiving frequency converter (down-converter) 5, a receiving filter 3, and a transmit/receive sharing unit 4. In FIG. 31, numeral 203 represents a transmit/receive antenna.
In this type of the radio transmitter/receiver, in order to execute a loop testing of the indoor equipment 200 and outdoor equipment 201, an examiner must go to the test field to install looping means between the terminal A and B of the indoor equipment 200 and to install shifter looping means between the terminals C (or C') and D (or D') of the outdoor equipment 201.
However, such a loop testing method takes much time because an examiner must go to every test spot to install the loop testing means.
For that reason, a loop testable radio transmitter/receiver, as shown in FIG. 28, has been proposed. The radio transmitter/receiver is formed of a transmission system TS, a receiving system RS, a transmit/receive sharing unit 4, and a looping component (RF LOOP) 6. The transmission system TS has an up-converter (transmission frequency converting means (TCONV)) 1, and a transmission filter (transmission signal filter means (TBF) 2. The receiving system RS has a receiving filter (receiving signal filter means (RBF)) 3, and a down-converter (receiving frequency converting means (RCONV)) 5.
The up-converter 1 converts a transmission intermediate frequency signal (transmission IF signal) from the modulator/demodulator (MODEM) 100 into a transmission frequency signal (transmission RF signal) and is formed of a local oscillator 11, a mixer 12, and a high-power amplifier (HPA) 13.
The transmission filter 2 suppresses an undesired wave contained in the transmission frequency signal from the up-converter 1. The receiving filter 3 suppresses an undesired wave contained in the receiving RF signal. A band pass filter is used for the filters 2 and 3.
The down-converter 51converts a signal from the receiving filter 3 a receiving intermediate frequency signal (receiving IF signal) to send the modulator/demodulator 100. The converter 5 includes a local oscillator 51, a mixer 52, and a low noise amplifier (LNA) 53.
The transmit/receiver sharing unit 4 outputs a transmission signal from the transmission system TS to the antenna 203 while it outputs a receiving signal from the antenna 203 to the receiving system RS. The looping component 6 converts the transmission signal of the transmission system TS into a receiving frequency signal and attenuates the outcome to loop back to the receiving system RS. The looping component 6 includes an attenuator 61, a RF change-over switches 62 and 63, a shifter 64, and a filter 65.
In this case, since it is better to separate off a failed device unit by looping at a place near to the output terminal of the equipment, the looping of the radio transmitter/receiver must be performed in the RF band. The looping component 6 is, as shown in FIG. 28, is constructed so as to loop back from the output of the high-power amplifier 13 to the input of the low noise amplifier 53. An ideal signal looping point is the point E shown in FIG. 28. However, since the failure rate of the passive elements such as the filters 2 and 3, the sharing unit 4, and the like is extremely small, the looping point is set at the E spot.
The attenuator (resistor) 61 attenuates the loop signal to the same level as that of the receiving signal. The RF change-over switch 62 switches a transmission signal to either the looping side or the transmission side. The RF change-over switch 63 switches to a loop signal or a receiving signal. The RF change-over switches 62 and 63 are formed of a waveguide WG as shown in FIG. 29 or pin-diodes PD1 and PD2, as shown in FIG. 30. In FIG. 30, C1 to C3 represent a dc-cut capacitor, C4 and C5 represent a capacitor, and L1 to L3 represent a coil. Pulses in a repulsive phase are applied through the coils L1 and L2. A high-level signal can make the pin-diode PD1 conductive and also can make the pin-diode PD2 conductive.
The shifter 64, which includes a local oscillator 66 and a mixer 67, converts the transmission signal in the transmission system TS into a a receiving frequency signal. The filter 65 is a sharp response filter for separating a transmission frequency signal from a receiving frequency signal.
In the above configuration, the up-converter 1 converts the transmission IF signal from the outdoor modulator/demodulator 100 into a frequency signal and the high-power amplifier 13 amplifies it to a desired output level. The amplified signal is supplied as a transmission output signal to the antenna feeding end via the transmit/receive sharing unit 4.
The receiving signal is amplified sufficiently by the low noise amplifier 53, subjected to a frequency conversion by the down-converter 5, and then transmitted as a receiving IF signal to the modulator/demodulator 100.
In the case of a loop testing, the looping unit 6 converts a transmission signal into a signal of a receiving frequency, attenuates further the resultant signal, and then loops back it to the receiving system RS, whereby it can be judged whether the devices before the looping point are normal or abnormal.
However, since being formed as a high frequency circuit, the looping component 6 shown in FIG. 28 is costly structurally and materially. Particularly, the looping unit used over microwave or submillimeter wave band is very expensive.
Since the switch 62 arranged at the output of the high power amplifier 13 and the switch 63 arranged at the input of the low noise amplifier 53 cause an insertion loss, a waveguide-type switch (shown in FIG. 29) is often used to reduce the loss. The waveguide switch is very expensive and large in structure and occupies a large space in the whole equipment.
Furthermore, the conventional equipment requires a local oscillator 66 for loop frequency conversion which has good frequency stability and good C/N (carrier to noise) ratio characteristics. Generally, A sharp response filter 65 is needed to the output of the shifter 64 because the transmission frequency is relatively close to the receiving frequency.
As described above, there are disadvantages in that forming a return loop in the RF band leads to a costly, bulky radio transmitter/receiver.